Method for measuring link quality in a wireless communication system and apparatus therefor

ABSTRACT

The present invention relates to a method for reporting the link quality of a downlink by a terminal. Specifically, the method includes the steps of: receiving, from a serving cell, a subframe set for resource-limited measurement and information regarding a cell-specific reference signal of an interference cell; measuring link quality of a downlink in the subframe set; and reporting the measured link quality of the downlink to the serving cell, wherein interference control processing due to the cell-specific reference signal from the interference cell is applied to the subframe set using the information regarding the cell-specific reference signal of the interference cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2012/007061, filed on Sep. 4, 2012,which claims the benefit of U.S. Provisional Application No. 61/537,034,filed on Sep. 20, 2011, 61/582,800, filed on Jan. 3, 2012, and61/645,600, filed on May 10, 2012, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method for measuring link quality in a wirelesscommunication system, and an apparatus therefor.

BACKGROUND ART

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd generation partnership project long termevolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a diagram schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An evolved universalmobile telecommunications system (E-UMTS) is an advanced version of alegacy universal mobile telecommunications system (UMTS) and basicstandardization thereof is currently underway in 3GPP. E-UMTS may begenerally referred to as an LTE system. For details of the technicalspecifications of UMTS and E-UMTS, reference can be made to Release 7and Release 8 of “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a user equipment (UE), evolvedNode Bs (eNode Bs or eNBs), and an access gateway (AG) which is locatedat an end of an evolved UMTS terrestrial radio access network (E-UTRAN)and connected to an external network. The eNBs may simultaneouslytransmit multiple data streams for a broadcast service, a multicastservice, and/or a unicast service.

One or more cells are present per eNB. A cell is configured to use oneof bandwidths of 1.25, 2.5, 5, 10, 15, and 20 MHz to provide a downlinkor uplink transmission service to multiple UEs. Different cells may beconfigured to provide different bandwidths. The eNB controls datatransmission and reception to and from a plurality of UEs. Regardingdownlink (DL) data, the eNB transmits DL scheduling information tonotify a corresponding UE of a time/frequency domain within which datais to be transmitted, coding, data size, and hybrid automatic repeat andrequest (HARQ)-related information by transmitting DL schedulinginformation to the UE. In addition, regarding uplink (UL) data, the eNBtransmits UL scheduling information to a corresponding UE to inform theUE of an available time/frequency domain, coding, data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic between eNBs may be used. A core network (CN) mayinclude the AG and a network node for user registration of the UE. TheAG manages mobility of a UE on a tracking area (TA) basis, each TAincluding a plurality of cells.

Although radio communication technology has been developed up to LTEbased on wideband code division multiple access (WCDMA), demands andexpectations of users and providers continue to increase. In addition,since other radio access technologies continue to be developed, newadvances in technology are required to secure future competitiveness.For example, decrease of cost per bit, increase of service availability,flexible use of a frequency band, a simplified structure, an openinterface, appropriate power consumption of a UE, etc. are required.

DETAILED DESCRIPTION OF THE INVENTION Technical Problems

Based on the above-described discussion, the present invention isdevised to propose a method for measuring link quality in a wirelesscommunication system and an apparatus therefor.

Technical Solutions

According to an aspect of the present invention, provided herein is amethod for reporting downlink link quality by a user equipment in awireless communication system, including receiving information about asubframe set for resource restricted measurement and information about acell-specific reference signal of an interfering cell from a servingcell; measuring downlink link quality in the subframe set; and reportingthe measured downlink link quality to the serving cell, whereinprocessing for controlling interference caused by the cell-specificreference signal of the interfering cell is applied in the subframe setby using the information about the cell-specific reference signal of theinterfering cell.

A subframe included in the subframe set may be an almost blank subframe(ABS) or a multicast broadcast single frequency network ABS. Themeasuring may include measuring the downlink link quality under theassumption that interference caused by the cell-specific referencesignal from the interfering cell has been cancelled.

The downlink link quality may include first information and secondinformation corresponding to the first information and, when the secondinformation is transmitted multiple times between transmission periodsof the first information, a subframe for measuring the first informationand a subframe for measuring the second information may be assumed assubframes belonging to the subframe set. The first information may be arank indicator (RI) and the second information may include at least oneof a precoding matrix index (PMI) and a channel quality indicator (CQI).

The information about the subframe set may be received through a radioresource control (RRC) layer.

According to another aspect of the present invention, provided herein isa user equipment in a wireless communication system, including areception module configured to receive information about a subframe setfor resource restricted measurement and information about acell-specific reference signal of an interfering cell from a servingcell; a processor configured to measure downlink link quality in thesubframe set; and a transmission module configured to report themeasured downlink link quality to the serving cell, wherein theprocessor applies processing for controlling interference caused by thecell-specific reference signal of the interfering cell in the subframeset by using the information about the cell-specific reference signal ofthe interfering cell.

Advantageous Effects

According to the embodiments of the present invention, a UE in awireless communication system can effectively measure link quality andcan report the measured link quality to an eNB.

Effects according to the present invention are not limited to what hasbeen particularly described hereinabove and other advantages notdescribed herein will be more clearly understood by persons skilled inthe art from the following detailed description of the presentinvention.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a UE and an E-UTRANbased on the 3GPP radio access network specification.

FIG. 3 is a diagram illustrating physical channels used in a 3GPP systemand a general signal transmission method using the same.

FIG. 4 is a diagram illustrating the structure of a radio frame used inan LTE system.

FIG. 5 is a diagram illustrating the structure of a DL radio frame usedin an LTE system.

FIG. 6 illustrates a situation in which dominant interference ispresent.

FIG. 7a and FIG. 7b are diagrams explaining the difference in CRStransmission depending upon whether an ABS is configured as an MBSFNsubframe.

FIG. 8 illustrates an example of comparing the impact of inter-cellinterference on each code block when one transport block is split into aplurality of code blocks.

FIG. 9 is a diagram illustrating a CSI calculation method of a UEaccording to an embodiment of the present invention.

FIG. 10 illustrates an example of configuring a reference resource forPMI/CQI reporting to maintain consistency between RI reporting andPMI/CQI reporting according to an embodiment of the present invention.

FIG. 11 is a block diagram of a communication apparatus according to anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments which will be described hereinbelow are examplesin which technical features of the present invention are applied to a3GPP system.

Although the embodiments of the present invention will be describedbased on an LTE system and an LTE-advanced (LTE-A) system, the LTEsystem and the LTE-A system are purely exemplary and the embodiments ofthe present invention can be applied to any communication systemcorresponding to the aforementioned definition. In addition, althoughthe embodiments of the present invention will be described based onfrequency division duplexing (FDD), the FDD mode is purely exemplary andthe embodiments of the present invention can easily be applied tohalf-FDD (H-FDD) or time division duplexing (TDD) with somemodifications.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a UE and an E-UTRANbased on 3GPP radio access network specifications. The control planerefers to a path used for transmission of control messages, which isused by the UE and the network to manage a call. The user plane refersto a path in which data generated in an application layer, e.g. voicedata or Internet packet data, is transmitted.

A physical layer of a first layer provides an information transferservice to an upper layer using a physical channel. The physical layeris connected to a media access control (MAC) layer of an upper layer viaa transmission channel. Data is transmitted between the MAC layer andthe physical layer via the transmission channel. Data is alsotransmitted between a physical layer of a transmitter and a physicallayer of a receiver via a physical channel. The physical channel usestime and frequency as radio resources. Specifically, the physicalchannel is modulated using an orthogonal frequency division multipleAccess (OFDMA) scheme in DL and is modulated using a single-carrierfrequency division multiple access (SC-FDMA) scheme in UL.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of an upper layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Thefunction of the RLC layer may be implemented by a functional blockwithin the MAC layer. A packet data convergence protocol (PDCP) layer ofthe second layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IPv4 or IPv6 packet in a radiointerface having a relatively narrow bandwidth.

A radio resource control (RRC) layer located at the bottommost portionof a third layer is defined only in the control plane. The RRC layercontrols logical channels, transmission channels, and physical channelsin relation to configuration, re-configuration, and release of radiobearers. A radio bearer refers to a service provided by the second layerto transmit data between the UE and the network. To this end, the RRClayer of the UE and the RRC layer of the network exchange RRC messages.The UE is in an RRC connected mode if an RRC connection has beenestablished between the RRC layer of the radio network and the RRC layerof the UE. Otherwise, the UE is in an RRC idle mode. A non-accessstratum (NAS) layer located at an upper level of the RRC layer performsfunctions such as session management and mobility management.

One cell constituting an eNB is configured to use one of bandwidths of1.25, 2.5, 5, 10, and 20 MHz to provide a DL or UL transmission serviceto a plurality of UEs. Different cells may be configured to providedifferent bandwidths.

DL transmission channels for data transmission from the network to theUE include a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting paging messages,and a DL shared channel (SCH) for transmitting user traffic or controlmessages. Traffic or control messages of a DL multicast or broadcastservice may be transmitted through the DL SCH or may be transmittedthrough an additional DL multicast channel (MCH). Meanwhile, ULtransmission channels for data transmission from the UE to the networkinclude a random access channel (RACH) for transmitting initial controlmessages and a UL SCH for transmitting user traffic or control messages.Logical channels, which are located at an upper level of thetransmission channels and are mapped to the transmission channels,include a broadcast control channel (BCCH), a paging control channel(PCCH), a common control channel (CCCH), a multicast control channel(MCCH), and a multicast traffic channel (MTCH).

FIG. 3 is a diagram illustrating physical channels used in a 3GPP systemand a general signal transmission method using the same.

When power is turned on or the UE enters a new cell, the UE performs aninitial cell search procedure such as acquisition of synchronizationwith an eNB (S301). To this end, the UE may adjust synchronization withthe eNB by receiving a primary synchronization channel (P-SCH) and asecondary synchronization channel (S-SCH) from the eNB and acquireinformation such as a cell identity (ID). Thereafter, the UE may acquirebroadcast information within the cell by receiving a physical broadcastchannel from the eNB. In the initial cell search procedure, the UE maymonitor a DL channel state by receiving a downlink reference signal (DLRS).

Upon completion of the initial cell search procedure, the UE may acquiremore detailed system information by receiving a physical downlinkcontrol channel (PDCCH) and receiving a physical downlink shared channel(PDSCH) based on information carried on the PDCCH (S302).

Meanwhile, if the UE initially accesses the eNB or if radio resourcesfor signal transmission to the eNB are not present, the UE may perform arandom access procedure (S303 to S306) with the eNB. To this end, the UEmay transmit a specific sequence through a physical random accesschannel (PRACH) as a preamble (S303 and S305) and receive a responsemessage to the preamble through the PDCCH and the PDSCH associated withthe PDCCH (S304 and S306). In the case of a contention-based randomaccess procedure, the UE may additionally perform a contentionresolution procedure.

After performing the above procedures, the UE may receive a PDCCH/PDSCH(S307) and transmit a physical uplink shared channel (PUSCH)/physicaluplink control channel (PUCCH) (S308), as a general UL/DL signaltransmission procedure. Especially, the UE receives downlink controlinformation (DCI) through the PDCCH. The DCI includes controlinformation such as resource allocation information for the UE and hasdifferent formats according to use purpose thereof.

Meanwhile, control information that the UE transmits to the eNB on UL orreceives from the eNB on DL includes a DL/UL acknowledgment/negativeacknowledgment (ACK/NACK) signal, a channel quality indicator (CQI), aprecoding matrix index (PMI), a rank indicator (RI), and the like. Inthe 3GPP LTE system, the UE may transmit the control information such asCQI/PMI/RI through a PUSCH and/or a PUCCH.

FIG. 4 is a diagram illustrating the structure of a radio frame used inan LTE system.

Referring to FIG. 4, the radio frame has a length of 10 ms(327200×T_(s)) and includes 10 equal-sized subframes. Each of thesubframes has a length of 1 ms and includes two slots. Each slot has alength of 0.5 ms (15360 T_(s)). In this case, T_(s) denotes a samplingtime represented by T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns).Each slot includes a plurality of OFDM symbols in the time domain andincludes a plurality of resource blocks (RBs) in the frequency domain.In the LTE system, one RB includes 12 subcarriers×7 (or 6) OFDM symbols.A transmission time interval (TTI), which is a unit time for datatransmission, may be determined in units of one or more subframes. Theabove-described structure of the radio frame is purely exemplary andvarious modifications may be made in the number of subframes included ina radio frame, the number of slots included in a subframe, or the numberof OFDM symbols included in a slot.

FIG. 5 is a diagram illustrating control channels contained in a controlregion of one subframe in a DL radio frame.

Referring to FIG. 5, one subframe includes 14 OFDM symbols. The first tothird ones of the 14 OFDM symbols may be used as a control region andthe remaining 11 to 13 OFDM symbols may be used as a data region,according to subframe configuration. In FIG. 5, R1 to R4 representreference signals (RSs) or pilot signals for antennas 0 to 3,respectively. The RSs are fixed to a predetermined pattern within thesubframe irrespective of the control region and the data region. Controlchannels are allocated to resources unused for RSs in the controlregion. Traffic channels are allocated to resources unused for RSs inthe data region. The control channels allocated to the control regioninclude a physical control format indicator channel (PCFICH), a physicalhybrid-ARQ indicator channel (PHICH), a physical downlink controlchannel (PDCCH), etc.

The PCFICH, physical control format indicator channel, informs a UE ofthe number of OFDM symbols used for the PDCCH in every subframe. ThePCFICH is located in the first OFDM symbol and is configured withpriority over the PHICH and the PDCCH. The PCFICH is composed of 4resource element groups (REGs) and each of the REGs is distributed overthe control region based on a cell ID. One REG includes 4 resourceelements (REs). An RE indicates a minimum physical resource defined asone subcarrier by one OFDM symbol. The PCFICH value indicates values of1 to 3 or values of 2 to 4 depending on bandwidth and is modulated usingquadrature phase shift keying (QPSK).

The PHICH, physical hybrid-ARQ indicator channel, is used to carry aHARQ ACK/NACK signal for UL transmission. That is, the PHICH indicates achannel through which DL ACK/NACK information for UL HARQ istransmitted. The PHICH includes one REG and is cell-specificallyscrambled. The ACK/NACK signal is indicated by 1 bit and is modulatedusing binary phase shift keying (BPSK). The modulated ACK/NACK signal isspread with a spreading factor (SF) of 2 or 4. A plurality of PHICHsmapped to the same resource constitutes a PHICH group. The number ofPHICHs multiplexed to the PHICH group is determined depending on thenumber of spreading codes. The PHICH (group) is repeated three times toobtain diversity gain in the frequency domain and/or the time domain.

The PDCCH is allocated to the first n OFDM symbols of a subframe. Inthis case, n is an integer equal to or greater than 1, indicated by thePCFICH. The PDCCH is composed of one or more control channel elements(CCEs). The PDCCH informs each UE or UE group of information associatedwith resource allocation of transmission channels, that is, a pagingchannel (PCH) and a downlink shared channel (DL-SCH), UL schedulinggrant, HARQ information, etc. The PCH and the DL-SCH are transmittedthrough a PDSCH. Therefore, the eNB and the UE transmit and receive datathrough the PDSCH except for particular control information or servicedata.

Information indicating to which UE or UEs PDSCH data is to betransmitted and information indicating how UEs should receive and decodethe PDSCH data are transmitted on the PDCCH. For example, assuming thata cyclic redundancy check (CRC) of a specific PDCCH is masked by a radionetwork temporary identity (RNTI) ‘A’ and information about datatransmitted using a radio resource ‘B’ (e.g. frequency location) andusing DCI format ‘C’, i.e. transport format information (e.g. atransport block size, a modulation scheme, coding information, etc.), istransmitted in a specific subframe, a UE located in a cell monitors thePDCCH using RNTI information thereof. If one or more UEs having RNTI ‘A’are present, the UEs receive the PDCCH and receive a PDSCH indicated by‘B’ and ‘C’ based on the received information of the PDCCH.

Hereinbelow, a description of channel state information (CSI) reportingwill be given. In the current LTE standard, a MIMO transmission schemeis categorized into open-loop MIMO operated without CSI and closed-loopMIMO operated based on CSI. Especially, according to the closed-loopMIMO system, each of the eNB and the UE may be able to performbeamforming based on CSI in order to obtain multiplexing gain of MIMOantennas. To acquire CSI from the UE, the eNB transmits RSs to the UEand commands the UE to feed back CSI measured based on the RSs through aPUCCH or a PUSCH.

CSI is divided into three types of information: an RI, a PMI, and a CQI.First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of signal-to-interference plus noise ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

Meanwhile, as a method for mitigating inter-cell interference, acurrently discussed method is that an interfering cell uses an almostblank subframe (ABS) in which some physical channels are transmittedwith reduced transmit power or with no transmit power and an interferedcell schedules a UE in consideration of the ABS.

In this case, in terms of the UE of the interfered cell, an interferencelevel greatly varies with a subframe. In such a situation, in order toperform a more accurate radio link monitoring (RLM) operation, perform aradio resource management (RRM) operation for measuring reference signalreceived power (RSRP)/reference signal received quality (RSRQ), ormeasure the above-described CSI for link adaptation in each subframe,subframes in which the RLM/RRM and CSI are measured need to be limitedto a subframe set having uniform interference characteristics.

In the current LTE standard, the above discussion is reflected such thata UE is informed of a specific subframe set through higher layersignaling and RLM/RRM and CSI measurement is not performed in subframeswhich do not belong to the specific subframe set.

The present invention proposes a method in which a UE calculates andthen reports CSI or DL quality in a situation in which dominantinterference is present. Such a situation occurs when a UE is subject tointerference of a greater level than a signal of a serving cell thereof.This will now be described with reference to FIG. 6.

FIG. 6 illustrates a situation in which dominant interference ispresent.

As illustrated in FIG. 6, a situation in which a UE experience dominantinterference from a macro eNB may occur because the transmit power of apico eNB is lower than that of the macro eNB although the UE isconnected to the pico eNB which is nearest thereto.

For smooth operation of an interfered UE when dominant interference ispresent, an interference mitigation coordination operation may beperformed in which an interfering eNB (macro eNB in the example of FIG.6) stops transmission (or reduces transmit power) on partial time and/orfrequency resources and provides services to the UE on the interferencecancelled/reduced resources.

As an example, the macro eNB may configure some subframes as ABSs inwhich a unicast signal is not transmitted and transmit information aboutthe subframes to the pico eNB so that the pico eNB schedules the UE inthe interference reduced subframes.

Thus, if a specific eNB configures partial subframes as ABSs in order toreduce interference with a neighboring eNB, a PDCCH signal for unicastscheduling or a PDSCH signal is desirably not transmitted in subframesconfigured as ABSs. However, it is desirable to transmit some signalseven in the ABS in order to prevent incorrect operation of legacy UEsthat do not recognize the presence of the ABS. A representative signaltransmitted in an ABS is a cell-specific reference signal (CRS) forperforming measurement. However, the CRS transmitted even in the ARSfunctions as interference with respect to a UE of a neighboring cell,thereby causing performance degradation.

CRS transmission in the ABS depends upon whether the correspondingsubframe is configured as a multicast broadcast single frequency network(MBSFN) subframe in an interfering cell. The CRS is not transmitted in aPDSCH region when the subframe is configured as the MBSFN subframe.However, when the subframe is not configured as the MBSFN subframe, theCRS should be transmitted even in the PDSCH region.

FIGS. 7a and 7b are diagrams explaining the difference in CRStransmission depending upon whether an ABS is configured as an MBSFNsubframe. Particularly, in FIG. 7a , the ABS is not configured as theMBSFN and it can be appreciated that CRSs are transmitted in a PDSCHregion from the macro eNB. In FIG. 7b , however, the ABS is configuredas the MBSFN and it can be appreciated that the CRSs are not transmittedin the PDSCH region from the macro eNB.

While it is assumed in FIGS. 7a and 7b that CRSs of the pico eNB and themacro eNB are transmitted on the same subcarriers, the CRSs of the picoeNB and the macro eNB may be transmitted at different locations ofsubcarriers according to cell ID configuration because a value forfrequency shifting of a CRS depends on a cell ID.

To solve CRS interference in the ABS, a UE may perform appropriateprocessing. A representative example of processing is interferencecancellation in which the UE measures a CRS interference channel andrestores a desired signal by subtracting estimated interference from areceived signal. This scheme has an advantage of completely cancelingCRS interference in an ideal case but has a disadvantage in terms ofbattery consumption because signals of a neighboring cell should alwaysbe estimated.

Another example of processing is RE puncturing at a receiver. In thisscheme, the UE does not use REs subjected to strong interference fromCRSs of a neighboring cell upon performing decoding so as to avoid aninfluence of CRS interference. In spite of a disadvantage of beingincapable of using some REs for decoding, this scheme can be simplyachieved relative to the interference cancellation scheme.

The UE performing the above-described processing operation forovercoming CRS interference has a difficulty in guaranteeingcommunication reliability when an RI/PMI/CQI information calculationmethod or a link quality calculation method of the UE for CSI feedbackis not correctly indicated to an eNB. For example, the above-mentionedprocessing is desirably performed when there is strong CRS interferencefrom the macro eNB, whereas, it may be more effective not to perform theprocessing operation when CRS interference is not severe. In otherwords, the CRS interference processing of the UE is not always performedand is adaptively performed according to an interference situation.However, if the eNB is not aware of whether such processing is appliedat a specific time, the eNB cannot accurately judge the impact ofprocessing applied by the UE and thus there is difficulty in correctlyselecting a modulation and coding scheme (MCS) level.

More specifically, when interference cancellation is applied, since itis impossible to perfectly cancel interference in reality, a part of CRSinterference of a neighboring cell remains even after processing isperformed, thereby affecting decoding performance. The impact ondecoding performance differs according to the size of a transport block.This is because, if the size of one transport block exceeds apredetermined value, the transport block is split into a plurality ofcode blocks to be individually decoded and the occupation ratio of CRSinterference on specific REs to a specific code block is determined bythe size of the transport block. Generally, there is a high probabilitythat substantial CRS interference remains in a specific code block asthe size of the transport block increases because resources areallocated by a frequency first mapping scheme of an LTE PDSCH. Althougha network needs to be aware of the fact that interference cancellationis applied, proper link adaptation may be performed by estimating theimpact of CRS interference in the size of the allocated transport block.

FIG. 8 illustrates an example of comparing the impact of inter-cellinterference on each code block when one transport block is split into aplurality of code blocks.

Referring to FIG. 8, all allocated resources are split into three codeblocks but code block 1 experiences twice as much CRS interference asthe other code blocks. Therefore, even after interference cancellationis performed, decoding performance of this specific code block 1 isfurther degraded due to remaining CRS interference. Obviously, thisphenomenon does not appear when there is a small number of allocated RBsso that only one code block is present on all resources. Similarly,since the number of REs punctured in a specific code block is determinedby the size of a transport block even during RE puncturing of areceiver, it is necessary to discern which processing is operated inorder for a network to perform a link adaptation process. To solve sucha problem, the present invention proposes a method in which a UEmeasures CSI or link quality.

FIG. 9 is a diagram illustrating a method in which a UE measures CSI orlink quality according to an embodiment of the present invention.

First, a network may transmit a signal indicating whether to operate aprocess for handling CRS interference of a neighboring cell to a UE asshown in step 901. For instance, an eNB may command the UE to measureand report CSI or link quality, through higher layer signaling such asRRC signaling, under the assumption that the CRS interference handlingprocess has been activated as shown in step 903. Similarly, the eNB maycommand the UE to start to measure the CSI or link quality without sucha process.

Particularly, such an indication message may include information aboutCRS interference of the neighboring cell. Alternatively, the informationabout CRS interference may be provided as shown in step 902 separatelyfrom the indication message. The information about CRS interference mayinclude an ID of the neighboring cell, the number of antenna ports ofthe neighboring cell, a time/frequency offset value of a CRS RE, andMBSFN subframe configuration information of the neighboring cell.

Additionally, the eNB may inform the UE that the CSI or link quality isto be measured under a certain assumption about a processing type (i.e.assumption as to whether used processing is interference cancellation,receiver RE puncturing, or additional processing). Alternatively, the UEmay report, to the eNB, which type of processing is assumed to measurethe CSI or link quality.

In the case of aperiodic CSI reporting triggered by a PDCCH, anindicator indicating whether it is assumed that an MBSFN subframe of aspecific cell in a triggering PDCCH is configured may be added.

As a method for indicating an assumption about the CRS interferencehandling process during CSI or link quality measurement, a restrictedmeasurement message may be used as an implicit indicator. As describedabove, since the eNB configures resource restricted measurement forcorrect CSI or link quality measurement in a dominant interferenceenvironment, the UE having CRS interference handling capability mayinterpret such a resource restricted measurement configuration as asignal for activating the CRS interference handling process and applythe process included therein during PDSCH/PDCCH demodulation or CSI orlink quality measurement result feedback. In other words, if a subframeset for resource restricted measurement is configured, the UE mayactivate the above-described CRS interference handling process uponmeasuring CSI or link quality for each subframe set.

Even in this case, the eNB may transmit information about CRSinterference to the UE or the UE reports information about CRSinterference handled thereby to the eNB. Alternatively, withoutexchanging such information, the UE may be operated to report a resultof measuring CSI or link quality achievable after interference handlingunder the assumption that CRS interference of a uniform characteristicis always present (e.g. under the assumption that CRS interferencecorresponding to the number of specific antenna ports is present at aspecific location).

While the CSI or link quality measurement result based on such UEprocessing is fed back, the MBSFN subframe configuration of aninterfering cell affects UE feedback as illustrated in FIGS. 7a and 7b .Since CRS interference is present even in a PDSCH region of an ABS notconfigured as an MBSFN (hereinafter, referred to as a normal ABS) of aneighboring cell, the CSI or link quality measurement result afterinterference processing should be reported. On the other hand, since CRSinterference is not present in a PDSCH region of an ABS configured as anMBSFN (hereinafter, referred to as an MBSFN ABS) of the neighboringcell, it is proper to report the CSI or link quality measurement resultcalculated without additional CRS interference processing. In an actualnetwork operation situation, the normal ABS and the MBSFN ABS may bemixed and thus it is desirable to report an accurate CSI or link qualitymeasurement result by appropriately processing the normal ABS and theMBSFN ABS.

As one method to this end, the present invention proposes that a normalABS and an MBSFN ABS be not included in the same subframe set. That is,if specific subframes constitute one subframe set, all MBSFN subframesof an interfering cell in the specific subframes are identicallyconfigured. Then, the UE may measure CSI or link quality under theassumption that the same CRS interference handling process is performedin the same subframe set. For example, during PDSCH decoding, the eNBmay indicate a subframe set in which CRS interference is present and asubframe set in which CRS interference is not present, through ahigher-layer signal such as an RRC signal.

Alternatively, the UE may monitor presence/absence of CRSs of aneighboring cell and measure CSI or link quality under the assumption ofthe CRS interference handling process in a specific subframe set. Inthis case, a network preferably appropriately controls an MBSFN subframeconfiguration so that the UE may assume that the same CRS interferencecharacteristic is maintained in the same subframe set.

As another method, the present invention proposes that CSI or linkquality measurement be performed by applying a specific assumption ofthe UE about CRS interference to all subframes in a specific subframeset in a situation in which a normal ABS and an MBSFN ABS coexist in thesame subframe set. For instance, when the UE measures the CSI or linkquality under the assumption CRS interference is solved through one ofinterference cancellation or receiver RE puncturing, the UE reports aCSI or link quality measurement result achievable under the aboveassumption regardless of whether CRS interference is actually present inall subframes in the specific subframe set. In addition, assuming thatCRS interference is present, the UE may be operated to measure CSI orlink quality under the assumption that CRS interference is presentirrespective of whether CRS interference has actually been monitored inthe same subframe set.

Especially, such an operation suits periodic CSI reporting. In the caseof periodic CSI reporting, an RI is determined based on one subframe asa reference resource and then a PMI/CQI is determined based on anothersubframe as the reference resource. Here, even when presence/absence ofCRS interference on the reference resource during RI determination isnot equal to presence/absence of CRS interference on the referenceresource during PMI/CQI determination, consistent CSI measurement can beperformed. A reverse operation may also be performed. That is, the UEmay calculate CSI under the assumption that CRS interference is alwaysabsent irrespective of whether CRS interference is actually monitored insubframes and report the calculated CSI.

Additionally, CSI (or link quality measurement result) calculatedaccording to whether CRS interference is present or absent when thenormal ABS and the MBSFN ABS coexist in the same subframe set may beseparately fed back. In other words, CSI (or link quality measurementresult) measured under the assumption that CRS interference is presentand CSI (or link quality measurement result) measured under theassumption that CRS interference is absent are separately fed back so asto cause the network to provide information to be used in acorresponding subframe.

Alternatively, to maintain consistency between RI reporting and PMI/CQIreporting, the UE may be operated to perform PMI/CQI reporting under theassumption that the same CRS interference as on a reference resourceduring RI reporting corresponding to specific PMI/CQI reporting ispresent (and under the assumption that proper processing for handing theCRS interference is performed).

FIG. 10 illustrates an example of configuring a reference resource forPMI/CQI reporting to maintain consistency between RI reporting andPMI/CQI reporting according to an embodiment of the present invention.

Referring to FIG. 10, an RI reported in subframe (SF) #n+4 and PMI/CQIreported in SF #n+8 configure SF #n and SF #n+4 as reference resources,respectively.

According to the present invention, if CRS interference is not presentbecause SF #n is an MBSFN ABS, the UE calculates the PMI/CQI under theassumption that CRS interference is not present irrespective of an MBSFNsubframe configuration of an interfering cell in SF #n+4 and reports thecalculated PMI/CQI in SF #n+8. If CRS interference is present because SF#n is a normal ABS, the UE calculates the PMI/CQI under the assumptionthat CRS interference is present irrespective of the MBSFN subframeconfiguration of the interfering cell in SF #n+4 and reports thecalculated PMI/CQI in SF #n+8. In other words, it is assumed that theMBSFN subframe configuration of the interfering cell on a referenceresource of CSI reporting is the same as configuration of a subframeused as a reference resource during RI reporting assumed incorresponding CSI reporting.

In particular, such an operation is effective during CSI reportingassociated with an intermittently transmitted CSI-RS for channelestimation of a serving cell. For example, if there are a plurality ofperiodic CSI reporting instances that a CSI-RS transmission occurs onceand the RI and the PMI/CQI are reported before a next CSI-RStransmission, the UE may assume that a serving cell channel is invariantbetween the CSI reporting instances. Therefore, there is an advantage inthat the PMI/CQI obtained during RI calculation can be reported withoutthe need of calculating the PMI/CQI again. Such a scheme may be slightlychanged so that the UE may assume that reference resources of all CSIreporting instances appearing until a CSI-RS is transmitted after theCSI-RS is transmitted once have the same MBSFN subframe configuration asan MBSFN subframe configuration of a reference resource of the firstinstance.

Assumption that the same reference resource configuration as a referenceresource configuration during RI reporting is used during PMI/CQIcalculation to be reported may be commonly applied not only to an MBSFNsubframe of an interfering cell but also to factors affecting PMI/CQIcalculation (e.g. the number of REs for an RS, the number of availableOFDM symbols, and the number of REs).

The present invention also proposes an operation of calculating anadditional CSI value according to the size of a transport block (orcodewords) (or the number of RBs allocated to determine the size of atransport size) and reporting the CSI value. As described above, when anoperation such as interference cancellation or receiver RE puncturing isperformed, an impact of CRS interference remaining after the operationvaries with the size of a transport block even in the same SNRenvironment. To solve this, according to the present invention, it isproposed that the network inform the UE of information such as the sizeof a transport block or the number of allocated REs assumed in CSIcalculation, through a higher-layer signal such as an RRC signal orthrough an L1/L2 control signal.

Alternatively, the UE may report a plurality of CSI values calculatedwith respect to a plurality of RBs. For example, in the case of periodicCSI reporting, the UE may be operated to feed back CSI for a smallnumber of allocated RBs (e.g. 4 RBs) once and feed back CSI for a largenumber of allocated RBs (e.g. all RBs) next time.

The operation proposed in the present invention, in which the UEmeasures CSI or link quality by applying a specific assumption about CRSinterference to all subframes in a specific subframe set when the normalABS and the MBSFN ABS are included in the same subframe set, may beimportantly applied when an interfering cell performs reduced non-zeropower PDSCH transmission (hereinafter, referred to as NZP-ABS operation)rather than zero power PDSCH transmission (hereinafter, referred to asZP-ABS operation) in an ABS.

Referring to back to the example of FIG. 7, if the macro eNB operatesthe NZP-ABS in the case in which the CRSs of the macro eNB collide withthe CRSs of the pico eNB, a pico UE has difficulty in measuringinterference. For instance, when an additional interference measurementresource is not configured, the pico UE cancels the CRSs of the eNB andmeasures interference under the assumption that signals monitored in theCRS-cancelled positions are interference signals. However, when themacro eNB operates the normal ABS in the case where the CRSs of the picoeNB collide with those of the macro eNB as illustrated in FIG. 7, theCRSs of the macro eNB are included in interference measurement so thatinterference is not measured in an ABS situation but rather interferencesimilar to interference in a non-ABS situation may be measured.

If the pico UE has capabilities of cancelling the CRSs of the macro eNB,the pico UE may cancel the CRSs of the macro eNB and measureinterference. However, even in this case, if the macro eNB operates theNZP-ABS, since interference measured by the pico UE after the CRSs ofthe macro eNB are cancelled does not include an actual PDSCH power ofthe macro eNB (although greater than 0 due to the NZP-ABS, the power issignificantly reduced as compared to a non-ABS), incorrect measurementis still performed.

Meanwhile, if the macro eNB operates the MBSFN ABS, since the CRSs ofthe pico eNB do not collide with the CRSs of the macro eNB in a PDSCHregion, interference from the macro eNB can be directly measured afteronly the CRSs of the pico eNB are cancelled. Accordingly, to solve aninterference measurement error between the normal ABS (i.e. a non-MBSFNABS) and the MBSFN ABS, interference may be measured and CSI or linkquality may be measured, under the above-described assumption that allsubframes belonging to a specific subframe set are non-MBSFN (or MBSFN)ABSs.

Additionally, when the macro eNB operates the NZP-ABS, an “RS-to-PDSCHtransmission power ratio” of the macro eNB may be transmitted to thepico UE so as to measure interference in the normal ABS (especially whenthe CRSs of the macro eNB collide with the CRSs of the pico eNB). Then,the pico UE may estimate the amount of interference in the NZP-ABS ofthe macro eNB based on the transmitted transmission power ratio aftermeasuring the CRSs of the macro eNB. After cancelling both the CRSs ofthe pico eNB and the CRSs of the macro eNB, the pico UE may calculateinterference in the NZP-ABS by adding the amount of interferenceestimated from the macro eNB to the measured interference and measureCSI or link quality based on the calculated interference. This operationmay be performed based on the CRSs of the macro eNB transmitted in botha PDCCH region and a PDCCH region in the normal ABS. On the other hand,if the CRSs of the pico eNB are cancelled from the PDSCH region in theMBSFN ABS as described above, interference in the NZP-ABS of the macroeNB may directly be measured.

Generally, the interference of the macro eNB directly measured in theMBSFN ABS is different from interference estimated by the pico UE in thenormal ABS due to traffic load or beamforming operation of the macro eNBand thus inconsistency of interference measurement between the normalABS and the MBSFN ABS occurs again. Even in this case, such a problemcan be resolved by applying a proper assumption about the presence ofCRS interference proposed in the present invention.

As an example, even when a specific subframe is an MBSFN ABS and thepico UE can directly measure interference of a macro eNB in a PDSCHregion, if the “RS-to-PDSCH transmission power ratio” of the macro eNBhas been transmitted for interference measurement in a normal ABS, anoperation for inducing an interference estimate in the normal ABS underthe assumption that a corresponding subframe is a non-MBSFN ABS, i.e. anoperation of calculating an interference estimate from the macro eNBusing a measurement value of the CRSs of the macro eNB and informationabout the transmitted power ratio under the assumption that the CRSs ofthe macro eNB are present, may be performed and CSI or link quality maybe measured based on such an operation, thereby solving theabove-described interference measurement inconsistency problems.

In performing this operation, since the macro eNB does not transmit CRSsin the PDSCH region of the MBSFN ABS, the pico UE may measure the CRSstransmitted by the macro eNB in the PDCCH region and perform the aboveoperation under the assumption that the macro eNB transmits CRSs of thesame signal size even in the PDSCH region based on the calculated CRSs(and under the assumption that a PDSCH is transmitted according to thetransmitted “RS-to-PDSCH transmission power ratio”).

As another example, the UE may be operated to measure CSI or linkquality by regarding only interference measured in the MBSFN ABS (morespecifically, measured in the PDSCH region of the MBSFN ABS without CRSsof the macro eNB) as effective. Particularly, this means that, althoughthe normal ABS is configured as the reference resource of the CSI, thepico UE may calculate the CSI based on interference measured in asubframe other than the reference resource, more specifically, measuredin an MBSFN ABS belonging to the same CSI subframe set as the referenceresource. In addition, to select one of two measurement schemes asnecessary, the eNB may indicate which assumption is used to measure CSIor link quality in a specific subframe set through a higher-layer signalsuch as an RRC signal.

The above-described operation may be applied even to the case in whichinterference is measured in a non-ABS. As an example, when the pico UEmeasures interference in a non-ABS in the case in which the CRSs of themacro eNB collide with the CRSs of the pico eNB, since the pico UE maymeasure interference after cancelling only the CRSs of the pico eNB in anormal ABS, interference corresponding to the CRSs of the macro eNB ismeasured. On the other hand, in an MBSFN ABS, interference correspondingto the PDSCH of the macro eNB is measured. Hence, inconsistency occursin measured interference. To cancel such inconsistency, a properassumption about whether the CRSs of the macro eNB are transmitted maybe introduced.

For example, the UE may measure CSI or link quality by regarding theCRSs of the macro eNB as interference from the macro eNB aftercancelling only the CRSs of the pico eNB as in the normal ABS under theassumption that the macro eNB transmits CRSs in the PDSCH region even inthe MBSFN ABS. As in the above description, since the CRSs of the macroeNB are not actually present in the PDSCH region of the MBSFN ABS, theCRS measurement values of the macro eNB in a PDCCH region may replacethose in the PDSCH region.

As still another example, since the pico UE is actually subject to muchinterference from the PDSCH of the macro eNB in the non-ABS, the pico UEmay be operated to measure CSI or link quality by regarding onlyinterference measured in the MBSFN ABS (more specifically, measured inthe PDSCH region of the MBSFN ABS in which the CRSs of the macro eNB arenot present) as effective. Especially, this means that, although thenormal ABS is configured as the reference resource of the CSI, the picoUE may measure the CSI or link quality based on interference measured ina subframe other than the reference resource, more specifically,measured in an MBSFN ABS belonging to the same subframe set as thereference resource.

FIG. 11 is a block diagram of a communication apparatus according to anembodiment of the present invention.

Referring to FIG. 11, a communication device 1100 includes a processor1110, a memory 1120, a radio frequency (RF) module 1130, a displaymodule 1140, and a user interface module 1150.

The communication device 1100 is illustrated for convenience ofdescription and some modules may be omitted. The communication device1100 may further include necessary modules. Some modules of thecommunication device 1100 may be further divided into sub-modules. Theprocessor 1100 is configured to perform operations according to theembodiments of the present invention exemplarily described withreference to the drawings. Specifically, for a detailed description ofoperations of the processor 2100, reference may be made to thestatements described with reference to FIGS. 1 to 10.

The memory 1120 is connected to the processor 1110 and stores operatingsystems, applications, program code, data, and the like. The RF module1130 is connected to the processor 1110 and performs a function ofconverting a baseband signal into a radio signal or converting a radiosignal into a baseband signal. For this, the RF module 1130 performsanalog conversion, amplification, filtering, and frequency upconversionor performs inverse processes thereof. The display module 1140 isconnected to the processor 1110 and displays various types ofinformation. The display module 1140 may include, but is not limited to,a well-known element such as a liquid crystal display (LCD), a lightemitting diode (LED), or an organic light emitting diode (OLED). Theuser interface module 1150 is connected to the processor 1110 and mayinclude a combination of well-known user interfaces such as a keypad anda touchscreen.

The above-described embodiments are combinations of elements andfeatures of the present invention in a predetermined manner. Each of theelements or features may be considered selective unless mentionedotherwise. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. In the appendedclaims, claims that are not explicitly dependent on each other may ofcourse be combined to provide an embodiment or new claims can be addedthrough amendment after the application is filed.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, orcombinations thereof. In the case of a hardware configuration, theembodiments of the present invention may be implemented by one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In the case of a firmware or software configuration, the methodaccording to the embodiments of the present invention may be implementedby a module, a procedure, or a function, which performs functions oroperations described above. For example, software code may be stored ina memory unit and then may be executed by a processor. The memory unitmay be located inside or outside the processor to transmit and receivedata to and from the processor through various well-known means.

The present invention may be carried out in other specific ways thanthose set forth herein without departing from the spirit and essentialcharacteristics of the present invention. The above embodiments aretherefore to be construed in all aspects as illustrative and notrestrictive. The scope of the invention should be determined by theappended claims and their legal equivalents and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

INDUSTRIAL APPLICABILITY

While the method and apparatus for measuring link quality in a wirelesscommunication system have been described in the context of a 3GPP LTEsystem, the present invention is also applicable to many other wirelesscommunication systems in addition to the 3GPP LTE system.

The invention claimed is:
 1. A method for performing a measurement by auser equipment in a wireless communication system, the methodcomprising: receiving, via a higher layer, information about one or moresubframe sets related to a neighbor cell and configuration informationabout a cell-specific reference signal of the neighbor cell; performinga resource restricted measurement on subframes which are indicated bythe information about the one or more subframe sets related to theneighbor cell; performing a resource unrestricted measurement onsubframes which are not indicated by the information about the one ormore subframe sets related to the neighbor cell; and transmitting aresult of the resource restricted measurement and a result of theresource unrestricted measurement, wherein the resource restrictedmeasurement is based on the configuration information about thecell-specific reference signal of the neighbor cell and includesmitigating an interference occurred by the cell-specific referencesignal of the neighbor cell on the subframes which are indicated by theinformation about the one or more subframe sets related to the neighborcell, and wherein the resource unrestricted measurement is performedwithout mitigating any interference occurred by the cell-specificreference signal of the neighbor cell on the subframes which are notindicated by the information about the one or more subframe sets relatedto the neighbor cell.
 2. The method according to claim 1, wherein thesubframes indicated by the information about the one or more subframesets related to the neighbor cell are almost blank subframes (ABSs) ormulticast broadcast single frequency network (MBSFN) ABSs.
 3. The methodaccording to claim 1, wherein the higher layer is a radio resourcecontrol (RRC) layer.
 4. A user equipment in a wireless communicationsystem, the user equipment comprising: a receiver configured to receive,via a higher layer, information about one or more subframe sets relatedto a neighbor cell and configuration information about a cell-specificreference signal of the neighbor cell; a processor configured to performa resource restricted measurement on the subframes which are indicatedby the information about the one or more subframe sets related to theneighbor cell and perform a resource unrestricted measurement onsubframes which are not indicated by the information about the one ormore subframe sets related to the neighbor cell; and a transmitterconfigured to report a result of the resource restricted measurement anda result of the resource unrestricted measurement, wherein the resourcerestricted measurement is based on the configuration information aboutthe cell-specific reference signal of the neighbor cell and includesmitigating an interference occurred by the cell-specific referencesignal of the neighbor cell on the subframes which are indicated by theinformation about the one or more subframe sets related to the neighborcell, and wherein the resource unrestricted measurement is performedwithout mitigating any interference occurred by the cell-specificreference signal of the neighbor cell on the subframes which are notindicated by the information about the one or more subframe sets relatedto the neighbor cell.
 5. The user equipment according to claim 4,wherein the subframes indicated by the information about the one or moresubframe sets related to the neighbor cell are almost blank subframes(ABSs) or multicast broadcast single frequency network (MBSFN) ABSs. 6.The user equipment according to claim 4, wherein the higher layer is aradio resource control (RRC) layer.